The invention relates to photographic cameras provided with a negative-feedback diaphragm control system of the type including a light sensor arranged behind a variable light attenuator. The negative-feedback diaphragm control system includes an electromagnetic drive, for instance a D.C. motor, which moves both the movable component(s) of the diaphragm per se and also the variable light attentuator, to thereby vary the attenuation of light incident upon the light sensor. During operation of the negative-feedback diaphragm control system, the system automatically moves both the movable component(s) of the diaphragm and the variable light attenuator arranged in front of the light sensor, in a manner automatically maintaining the intensity of light incident upon the light sensor substantially constant. There is additionally provided an automatic exposure-duration control system, which includes a threshold stage which changes state when its input signal reaches a threshold level, and a light-dependent timing stage connected to the input of the threshold stage. The timing stage applies to the threshold stage a changing input signal which varies in direction toward the threshold level at a rate dependent upon the prevailing brightness of scene light.
With the type of prior-art camera in question, within the range of regulation of the diaphragm control system, the diaphragm setting is automatically adjusted as a function of scene light. However, so long as the diaphragm control system is operating intermediate the limits of its range of adjustment, the amount of light incident upon the light detector of the exposure-duration control system remains constant, i.e., so that the automatically selected exposure duration does not vary as the diaphragm setting is being varied. The automatically selected exposure duration begins to be varied, only when one or the other limit of the range of adjustment of the diaphragm control system has been reached. In other words, the exposure duration selection remains unchanged so long as the diaphragm has not yet been set to its minimum or maximum setting, because a limit of the range of adjustment of the diaphragm control system has not been reached and continued variation of the diaphragm setting may yet bring the system into equilibrium. However, once the diaphragm has already been brought into its minimum or maximum setting, and the system is still not in equilibrium, then the exposure duration selection is changed, in an attempt to establish the conditions for a proper exposure in this way.
This type of diaphragm-setting/exposure duration program is disadvantageous. Considering first, second and third ranges of scene brightness values, the diaphragm setting is automatically varied but the exposure-duration selection not varied within the second range of scene brightness values, whereas in the neighboring first and third ranges of scene brightness values the exposure-duration selection is varied but not the diaphragm setting.